Oxidative damage, particularly to proteins, has been widely postulated to be a major causative factor in the loss of functional capacity during senescence. The nature of the various mechanism that may contribute to protein oxidation is only partially understood. In this study, concentrations of two markers for oxidative damage, o,o'ditryronsine and o-tyrosine, were determined using stable isotope dilution gas chromatography-mass spectrometry in four tissue of the mouse, namely heart, skeletal muscle, brain and liver during youth (4month old), adulthood (14 month old) and old (30 month old) age. A comparison was made between mice that had access to unlimited calories with those that were restricted to 60% of the caloric intake of the ad libitum regimen. Caloric restriction of this magnitude extends the average and maximum life-span of mice by about 40% IN vitro studies demonstrated that o,o'ditryronsine was generated selectively in proteins exposed to tyrosyl radical. o-Tyrosine increased in proteins oxidized with hydroxyl radical, which also resulted in a variable increase in o, o'dityrosine. In mice fed ad-libitum, levels of o,o'-dityrosine increased with age in cardiac and skeletal muscle but not in liver or brain. In contrast, o-tyronsine levels did not rise with age in any of the tissues examined. These results suggest that tyrosyl radical- radical-induced protein oxidation increases selectively with age in skeletal muscle but did not influence o-tyrosine levels in any of four tissue. This selective increase in o, o'-dityrosine levels and its prevention by life-prolonging caloric restriction regimen raises the possibility that oxidation of muscle protein by tyrosyl radical contributes to the deterioration of cardiac and skeletal muscle function with advancing age.